The invention relates to the technical sector relating to systems of drive transmission with cogged belts or similar cogged flexible organs, for example a harmonic steel sheet exhibiting fashioned cogs.
In the simplest configuration, these systems are constituted, as is known, by a pair of cogged pulleys, of which one is a drive pulley and the other a driven pulley, connected to a cogged belt or other similar organ, of a suitable length.
In accompanying FIGS. 1, 2, 3 relating to the prior art, portions of a cogged belt 10 are illustrated, with a cogged drive pulley; FIG. 4, also relating to the prior art, illustrates, similarly to FIG. 1, portions of the cogged drive pulley 1 and a cogged flexible organ 20 of the above-indicated type, formed by a sheet 21 to which cogs 22 are associated.
With a above-mentioned elementary configuration, simple drive transmission between the drive pulley 1 and the driven pulley (not illustrated) can be obtained, or the basic design of a belt conveyor can be defined, the active branch of which coincides with the stretched branch A (the upper branch in the illustrated examples, with a clockwise rotation direction W of the drive pulley 1).
In the case of the conveyor, known drawing elements such as plugs, walls, edges etc., arranged at regular steps (not illustrated) can be associated to the external surface of the cogged belt 10.
Normally, the enmeshing between the flexible transmission organ 10, 20 and the pulley, whether a drive pulley 1 or a driven pulley, is such that the top SP of cogs 2 thereof meets the bottom FT of the recesses 13, 23 of the organ 10, 20 (FIGS. 1 and 4); in this way the radius R with which the flexible transmission organ 10, 20 winds on the pulley 1 is calculated with certainty (FIGS. 1, 2, 3).
The top ST of the cogs 12, 22 of the flexible transmission organ 10, 20 does not however meet the bottom FP of the recesses 3 of the pulley 1 (see FIGS. 1 and 4 again).
The dimension of the recesses 3 of the pulley 1 is established with a field of tolerance such that in any case the cogs 12, 22 of the flexible transmission organ 10, 20 can insert without interference on the sides, such as to guarantee the resting of the cogs 2 of the pulley 1 on the bottom FT of the chambers 13, 23 of the organ 10, 20 (FIGS. 1 and 4).
It is frequently found that there is a minimum of play between the recesses 3 of the pulley 1 and the cogs 12, 22 which couple with them (see FIGS. 1 and 4 once more).
In theoretical static conditions, each cog 12, 22 of the flexible transmission organ 10, 20 is considered to be centered with respect to the corresponding recess 3 of the pulley 1, such that the play is symmetrically distributed on the two sides (FIGS. 1 and 4).
The drawing of the flexible transmission organ 10, 20 following the starting-up of the drive pulley 1 is initially done by the friction existing in the contact zones between the top SP of the cogs 2 of the pulley 1 and the bottom FT of the recesses 13, 23 of the organ 10, 20.
The flexible transmission organ 10, 20 progressively tends to slide backwards with respect to the drive pulley 1, due to the resistant forces acting on the driven pulley or on the active branch A of the conveyor.
The first part of the sliding is absorbed by the play existing between the cogs 12, 22 of the flexible transmission organ 10, 20 and the recesses 3 of the pulley 1: when the flanks of the cogs and the recesses are in contact (FIG. 3) any further staggering between them is prevented, but not the tendency of the tops SP to slide with respect to the bottoms FT, so that when the pressure between the flanks increases, the drive torque of the pulley 1 consequently exerts, on the portion of the flexible transmission organ 10, 20 wound thereon, and consequently on the active branch A thereof, a sharp traction which pushes the cogs 12, 22 newly towards the centre of the recesses 3 of the pulley 1, with a consequent slippage in a same direction of the tops SP with respect to the bottoms FT.
The moment at which the pulley 1 returns the flexible organ 10, 20 forwards causes a sort of a clicking effect on the flexible organ 10, 20, which is cyclically repeated. This irregularity in advancement is detrimental, especially with regard to a belt conveyor, since it has a knock-on effect on the objects travelling thereon.
The above-described phenomenon is more evident the greater the interaxis between the drive pulley and the driven pulley, and consequently the length of the cogged flexible organ connecting them. The differences between the rated values and the real values of the cogging's primitive diameter and step also contribute to the appearance of the drawback, due to the possible shifts in the admissible tolerances.